@JohnRennie, well, it's a bit odd. To those who already know the answer, it's clearly not about physics. However, to someone who doesn't already know the answer, it's not inconceivable that a label consistently appearing in a set of physics books could have had some kind of physics-based significance. How do you treat a question that you can only identify with confidence as "not physics" if you already know the answer?
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user27578Mar 12 '14 at 15:47

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This question appears to be off-topic because it is about library science.
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David Z♦Mar 12 '14 at 16:05

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Cases like this are so rare that I don't know if we really need a formal policy. We can just leave this here, on hold, and people will be able to find it, but it'll still show up as not being on topic.
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David Z♦Mar 12 '14 at 16:06

2 Answers
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It's an identification number for a classification scheme called the Dewey Decimal System. It's just a general system most libraries use for organizing books by subject matter; the numbers don't mean anything. 500 is the Dewey series for science, 530 is physics, and 539 is modern physics.

I don't know if this question is really appropriate for Physics.SE, but then, I don't know if any other SE would be.

Yeah, where do you post this question? I searched google, no answer no question. At least now it has been asked and answered somewhere (here).
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Love LearningMar 12 '14 at 15:40

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I think it could be really useful to have a physicists.stackechange.com to ask questions that come up in the life of a physicist, but are not directly physics. Alternatively one could think about allowing them on meta.physics.stackexchange.com, which is right now only meta Q&A for the site itself.
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jdmMar 12 '14 at 15:55

Ahh yeah, our library just recently switched to these funny numbers, and then of course I started noticing these numbers in books, all over the place .
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Love LearningMar 12 '14 at 15:39

@LoveLearning, "just recently"? It seems surprising to me that a library would only now start using the standard for library organization of nonfiction books. Is it an especially small library?
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Brian SMar 12 '14 at 16:22

Perhaps they used it internally, but kept it hidden from our eyes.
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Love LearningMar 12 '14 at 16:24

How were you able to find books if they weren't labelled?
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user37496Mar 12 '14 at 16:54

The books themselves where labelled (with name and number I think), but the shelves where labelled with names (e.g. PhysTheo something something) rather than these numbers (if I recall correctly).
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Love LearningMar 12 '14 at 17:27